You get paid for clicks and impressions but impressions are based on CPM (cost per thousand). Earnings depend on the competition there is to have ads placed on your site.
CPM ads only appear when an AdWords advertiser has selected your site specifically to run an ad on. One of these ads are either image ads or expanded text ads, and they will always take up an entire ad unit. Earnings are based on competition and also on click-thru rate, as if CPC ads are earning more then they will display instead of the CPM ad.
it depends on the content of your website and your luck, very few advertisers go for pay per impression advertisment. i use adwords but i have personally never tried pay per impression advertising..